US President Donald Trump said on Sunday that Deputy Secretary of Defence, Patrick Shanahan, will assume the title of Acting Secretary of Defence starting January 1, accelerating the planned departure of Secretary of Defence James Mattis by two months.
Mr Trump's decision to accelerate Mr Mattis' departure comes days after US special envoy to the coalition fighting ISIS, Brett McGurk, also filed his resignation over the president's decision to withdraw troops from Syria.
Last week, Mr Trump said that Mr Mattis would retire at the end of February. But the president decided to speed-up the process because he was irked by the attention given to Mr Mattis' resignation letter, which implicitly criticised Mr Trump's foreign policies and his treatment of military allies, a source familiar with the situation told Reuters.
"He just wants a smooth, more quick transition and felt that dragging it out for a couple of months is not good,” the official said, on condition of anonymity.
The official said Mr Trump was expected to pick a nominee for defense secretary over the next couple of weeks.
In the meantime, Mr Mattis "will continue to focus on what he needs to do ... to ensure a smooth transition," a US defense official, also speaking on condition of anonymity, said.
Mr Mattis and others had strongly advised the president against a Syria exit and the retired general did little to hide his disagreement with Mr Trump.
"Because you have the right to have a secretary of defense whose views are better aligned with yours," Mr Mattis said in the letter, "I believe it is right for me to step down from my position."
"My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held and informed by over four decades of immersion in these issues," Mr Mattis wrote.
His replacement, Mr Shanahan, spent over three decades working for aircraft giant Boeing, including as vice president and general manager of Boeing Missile Defense Systems, before moving to the Pentagon as deputy in 2017.
He has also been a vocal booster within the Pentagon for Mr Trump’s Space Force. In November, he brushed back an Air Force estimate that standing up the force would cost $13 billion, saying it should cost much less.
The resignations of Mr Mattis and Mr McGurk, go further than personal protests against the president. They show the mismatch between long-standing US policy and the current occupant of the White House is no longer tenable.